In the chemical industry a common technique for purifying or analysing chemicals is an exchange process. Solvent extraction relies upon the preferential transfer of one or more components from one phase (fluid) in which the component (solute) is dissolved into a second immiscible phase. Usually this is accomplished by physical mixing followed by separation of the two phases using gravity. It has been found that the more thoroughly the two phases are mixed, the more rapidly the transfer process proceeds by reason of the greater surface area of the smaller globules of liquid and reduced diffusion distances within the phases. The time for separation of the phases however increases with more thorough mixing, and hence for a desired efficiency of solute transfer, the separation time may become unacceptably long, this being the principal disadvantage of the process.
Our copending International Application PCT/GB95/02489 discloses a method and means of bringing first and second immiscible fluids in contact with one another for interaction, while inhibiting physical mixing of the fluids, to permit easy separation of the fluids subsequent to interaction, and claims apparatus for carrying out a process between first and second immiscible fluids, the apparatus comprising first and second flow paths for permitting fluid flow of respective first and second immiscible fluids therethrough, portions of the flow paths being disposed close to or adjacent one another and communicating with one another in a region which is such as to permit the fluids to form a stable open interface therein, and wherein at least the first flow path in the interface region has a width normal to the interface within the range 10 to 500 .mu.m.
The Application also discloses a method of carrying out a process between first and second immiscible fluids, the method comprising:
1) providing first and second flow paths having portions disposed adjacent to or close to one another and communicating with one another in a region in which the fluids can contact one another; PA1 2) flowing the first and second immiscible fluids through respective said first and second flow paths such that, at least in said region, the flow of both fluids is essentially laminar, and a stable open interface is formed between the fluids; PA1 3) permitting significant transfer of a desired entity at said interface between the fluids by diffusive transport within the fluids; and PA1 4) flowing the fluids away from the interface region in their respective flow paths without mixing of the fluids.
In the interface region, the flow paths are close to or adjacent one another so that fluid flow through the flow paths continually replenishes the fluid at the interface.
An object of the present invention is to provide a means of stabilising the interface between the liquids
It has now been realised that, although the liquids contact one another for a relatively short time and over a relatively short interface length, nevertheless the properties such as density, viscosity, inter-facial tension and volumes of the two liquids may vary across the interface length in consequence of the continuous transfer of a solute, for example, and this may disturb the operation of transfer unless some means for compensation is provided.
An object of the present invention is to provide a means of compensating for the aforementioned disturbance of the diffusive transfer across the interface
The present invention is applicable to the formation of an open interface maintained by surface tension, or to an interface maintained by a porous membrane between the fluids, as for example described in our copending International Application PCT/GB95/02488.
According to one aspect of the present invention there is provided an apparatus for carrying out a process between first and second immiscible fluids, the apparatus comprising first and second channels defining flow paths for permitting fluid flow of respective first and second immiscible fluids therethrough, portions of the flow paths being disposed close to, or adjacent to, one another and communicating with one another to define a region in which, in use, a stable interface between the fluids is formed, further characterised in that one or more dimensions of the region is varied along the length of the interface in a direction of fluid flow to compensate for variation in the properties of the fluids during the carrying out of the process and thereby maintain stability of the interface.
The cross sectional area of at least the first flow path is changed and preferably the cross sectional area of at least the second flow path is also changed.
In the case of an apparatus having, an open interface formed between the flow paths,it is preferable that the height of the interface is varied along the length of the interface in the direction of fluid flow.
In the case where the interface is defined by a porous or foraminated sheet the dimensions of the apertures in the sheet are varied along the length of the interface.
Preferably, at least at said region where the interface is formed, the first and second channels merge to form one or more tubes, the, or each of which, has two walls which converge in a direction extending normal to the interface to define a tapering cross section extending in a direction away from the open interface to hold or trap the fluid therein.
Preferably the channels merge at said region to form a tube of triangular cross sectional shape, with one apex of the triangle defining the first fluid flow path and a base of the triangular shape defining the second flow path.
Alternatively the channels merge at said region to form a tube having a cross sectional shape comprising a plurality of pairs of walls which converge mutually in a direction extending radially outwards relative to a central axis of the tube to form an apex, said apices defining the first fluid flow paths and the second flow paths being formed by a void extending along said central axis of the tube. The tube may be of star shaped cross section.
According to another aspect of the present invention there is provided a method of carrying out a process between first and second immiscible fluids, the apparatus comprising first and second channels defining flow paths for permitting fluid flow of respective first and second immiscible fluids therethrough, portions of the flow paths being disposed close to, or adjacent to, one another and communicating with one another to define a region in which, in use, a stable interface between the fluids is formed, further characterised in that one or more dimensions of the region is varied along the length of the interface in a direction of fluid flow to compensate for variation in the properties of the fluids during the carrying out of the process and thereby maintain stability of the interface.
As described in our copending International Applications PCT/GB95/02489, and PCT/GB95/02488, the interface position may be set and stabilised at a narrowing in the opening between channels or pores. The height d of the narrow part of the opening affects the pressure difference which may be maintained between immiscible fluids across the interface. Generally the maximum pressure differential for stability is described by an expression of the form EQU (P.sub.1 -P.sub.2)&lt;(2.gamma..cos .theta.)/d
where P.sub.1, and P.sub.2 are pressures in the immiscible fluids 1 and 2 either side of the interface, y is the interfacial tension, and .theta. is the value of an angle containing the contact angle between the interface and wall material, and an angle between the wall and plane of the opening.
The carrying out of the process may also result in changes of surface tension properties of the fluids. Thus there may be a net reduction in the surface tension between the fluids, whence a reduction of the height of the open interface (or a reduction in pore cross-section where an intervening membrane is employed) may be necessary. Conversely where a net increase in the interfacial surface tension occurs, an increase in opening height (or pore cross-section) may be allowed which will enhance material transfer.
When operating a stable open interface between immiscible fluids flowing in contacting flow paths it is necessary to limit the pressure differentials existing at all points along the interface to values compatible with the interface height, wall to interface contact angles, and the inter-fluid interfacial tension as set forth in International Application PCT/GB95/02489. It is not necessary nor in general desirable that the pressure differential across the interface is zero, but it must be maintained within bounds set by device geometry and material characteristics. During material transfer in the interface region the amounts of material within each phase changes and correspondingly the material properties such as density, viscosity, and interfacial tension will change. This will cause the rate of pressure drop along each contacting channel containing immiscible fluids to vary differently. The ability of a pinned interface to deform and to accommodate a range of pressure differentials, allows stable flow without break up of the interface to occur; this is provided that the differences in pressure drop variation are not so great as to produce excessive pressure differentials across the interface at any point within the contactor. However where enough material transfer between fluids occurs to modify fluid volumes and material properties sufficiently, and the channel cross section, interface height, and total mass flow rates remain fixed, it is clear that conditions leading to interface, and hence contactor, instability can occur. Sufficiently reducing total mass flow rates can arrest the onset of instability, but reduction in throughput is undesirable. In accordance with the invention, the flow path and/or contact region geometry along the contactor are varied to accommodate the changes in fluid volumes, viscosities, and interfacial tensions which occur along the contactor.
Total mass flow rates for contacting channels must be equal over any contacting length, but where material transfer between the immiscible fluids changes the fluid density, then both individual channel volumetric flow rates, and the total volumetric flow rates for contacting channels, will change over the contacting length. In order to maintain stability, it is desirable that the rate of pressure drop within the contacting channels be maintained nearly equal by widening or narrowing channels to accommodate the changes in volumetric flow rate and viscosity due to changes in fluid composition. Alternately, or additionally, the height of the contacting interface may be changed along the contactor to accommodate changes in the pressure differential produced across the interface along the channel contact region. The desired changes in device geometry for a particular inter-fluid exchange system may be determined experimentally, or may be calculated where sufficient information exists on the system.
A means of maintaining interface stability while allowing movement of the interface to accomodate changes in fluid volumes and properties resulting from contact of the fluids and material transfer between fluids, is provided by forming the interface in structures where walls defining the end of the interface approach one another at an angle. In accordance with the present invention under appropriate conditions, it is possible to stabilise the interface by defining one flow path as tapering away in cross section from the open interface so that the containing walls close together and effectively hold or trap the fluid which preferably wets the wall materials in the tapered region; in such an arrangement it is not necessary to provide a discontinuity between the two flow paths. Thus in one preferred form, the first and second flow paths are defined by a simple tube of triangular cross-section, with one flow path being located in the region of one apex of the triangle. Flow paths for the fluid preferentially wetting the wall materials may be established in each apex of the triangle or apices of other channel cross-section with well defined apices such as diamond or star shapes. It is not neccessary that such cross sections are symetrical. A series of such apices may be formed as part of grooves or channels etched or otherwise formed in a the surface of a plate or other solid body. Channels to carry the immiscible fluids may be formed by holding or bonding together such bodies with other such formed bodies or with bodies having plane surfaces.
Connections to the flow paths within the apices may be established using finer channels or pores such that the fluid which does not preferentially wet the wall materials is prevented by surface tension effects from entering those finer channels or pores.
Thus, the present invention provides apparatus for carrying out a process between first and second immiscible fluids, the apparatus comprising first and second flow paths for permitting fluid flow of respective first and second immiscible fluids there through, portions of the flow paths being disposed close to or adjacent one another and communicating with one another in a region which is such as to permit the fluids to form a stable interface therein, and wherein at least one flow path tapers away in cross section from the open interface such that containing walls hold or trap the fluid therein.